The invention relates to a process for the purification of aqueous solutions of low molecular weight polyhydroxyl compounds. It is known that low molecular weight polyhydroxyl compounds can be obtained by condensing formaldehyde in the presence of lead compounds (German Offenlegungsschrifts Nos. 2,714,084 and 2,721,186 corresponding substantially to U.S. Application Ser. Nos. 829,171; 829,167 and 829,173 all filed on Aug. 30, 1977) or calcium compounds (German Offenlegungsschrift No. 2,721,186 substantially corresponding to U.S. Application Ser. No. 829,173).
Low molecular weight polyhydroxyl compounds, which may be hydrogenated to reduce carbonyl groups to hydroxyl groups, are valuable chain extenders and crosslinkers for polyurethane plastics as described in U.S. Application Ser. Nos. 829,171 and 829,167. In addition, they may be converted into non-ionic surface-active compounds by esterification with long-chain aliphatic monocarboxylic acids or by ethoxylation, as described in K. Lindner's "Tenside", Vol. III, Wissenschaftliche Verlagsgesellschaft Stuttgart 1964, page 2336.
The processes described above require purified low molecular weight polyhydroxyl compounds. For example, calcium and/or lead compounds, which are present in the solutions of low molecular weight polyhydroxyl compounds from the process used for their production must be removed after the formaldehyde condensation and before or after the optional hydrogenation step.
It is known that low molecular weight polyhydroxyl compounds can be freed from lead compounds by precipitation in sulfate form using sulfuric acid (U.S. Application Ser. Nos. 829,171 and 829,167).